Abstract

We study the impact of excluded volume interactions on the orientation
statistics of chain segments in polymer gels, and show that NMR experiments
provide a direct and unique measure of excluded- volume effects on the chain
statistics. In particular we consider the tensor order parameter, which can be
expressed as the second Legendre polynomial of the segment orientation with
respect a fixed end-to-end distance vector and which is directly related to the
residual coupling constant obtained in NMR experiments. We provide analytical
results for the case of single chains in good solvent and for semi-dilute
solutions. Computer simulations using the bond fluctuation model are applied to
compare with the analytical predictions. Considering polymer gels at the
equilibrium state of swelling we predict a unique relation between the tensor
order parameter and the correlation length (blob size) of the gel. Experiments
applying multiple-quantum NMR methods to both end-linked and randomly
crosslinked polymer networks are in excellent agreement with this prediction.
The initial decay of the tensor order parameter as observed in experiments at
low and intermediate degrees of swelling can be explained as a solvent effect
without making additional assumptions about constraint release processes during
swelling.